1. Field of the Invention
The present invention relates to an induction type transducer and an electronic caliper, and in particular, a small-sized induction type transducer with high detection performance and an electronic caliper using the same transducer.
2. Description of the Related Art
Measuring equipments such as an electronic caliper have been widely used for measurement of the thickness or other physical dimensions of objects in the manufacturing industry. As the main component of an electronic caliper, a transducer has been used.
Among various transducers, a capacitance type transducer and an induction type transducer are generally known. In, the capacitance type transducer, a transmitting electrode and a receiving electrode are provided on a grid (slider), and a signal electrode is provided on a scale opposed to the grid. The transmitting electrode and the receiving electrode on the grid are capacitively coupled with the signal electrode on the scale. A drive signal is supplied to the transmitting electrode, and a detection signal which is generated at the receiving electrode in accordance with the relative positions of the grid and scale is processed by a processing circuit, whereby the movement or position of the grid with respect to the scale is detected.
Such a capacitance type transducer is suitable for use in a relatively clean and dry environment such as an inspection room or a design office, however, it cannot be used for dimensional measurement in an environment where the degree of pollution is relatively high such as a machine shop. In the case where a particulate substance such as metal particles and grinding powder or a fluid such as a cooling or cutting fluid exists, the particulate substance or the fluid enters between the signal electrode on the scale and the signal electrode or receiving electrode on the grid, and changes the capacitance between the signal electrode and the transmitting electrode or receiving electrode, resulting in detection failure.
On the other hand, in an induction type transducer, the relative positions of the grid and scale are detected based on the electromagnetic induction between them, so that this transducer has an advantage in that it can be used for dimensional measurement in an environment with a relatively high degree of pollution.
FIG. 6 shows the principle of measurement of the induction type transducer. As shown in (b) of FIG. 6, a grid (slider) 10 and a scale 12 are disposed so as to be opposite to each other. The grid 10 is provided with exciting coils 10a and 10b, and a detecting coil 10c. The detecting coil 10c is disposed between the exciting coils 10a and 10b. On the other hand, a scale coil 14 is formed on the scale 12, a magnetic flux is generated when supplying a current to the exciting coils 10a and 10b, and an induced current flows in the scale coil 14 on the scale 12 due to electromagnetic induction. Then, a magnetic flux is generated by the induced current in the scale coil 14, and by this magnetic flux, an induced current (induced voltage) is generated in the detecting coil 10c on the grid 10. Since the induced current (induced voltage) changes in accordance with the relative positions of the exciting coils 10a and 10b and scale coil 14, if the grid 10 is moved in the direction of the arrow in the figure with respect to the scale 12, as shown in (a) of FIG. 6, a periodic induced voltage V is generated in the detecting coil 10c. Therefore, by detecting the value of the induced voltage, the relative positions of the grid 10 and the scale 12 can be detected.
Even if a pollutant such as water or oil is mixed between the grid 10 and scale 12, the magnetic flux and the magnetic non-permeability do not change and influence the induced voltage, so that the relative positions can be detected with high accuracy even in an environment with a high degree of pollution.
On the other hand, FIG. 7 shows a principle for detection of the absolute displaced positions of the grid 10 and scale 12 by using the abovementioned principle. Herein, the absolute displaced positions mean the amounts of displacement from a certain reference point (zero point). As shown in (a) of FIG. 7, a plurality of exciting coils 10a are provided on the grid 10, and a plurality of detecting coils 10c are provided in accordance with these exciting coils. Scale coils 14a and 14b, whose center portions have a pitch of xcex1, and end portions have a pitch of xcex2 are formed on the scale 12. The pitch at the center portion and the pitch at the end portion are different from each other, so that two induced voltages of the pitches of xcex1 and xcex2 are also generated in the detecting coils 10c formed at the center portion and the end portion on the grid 10. Since one cycle of the two signals differs from each other, the relationship in the induced voltage between two wavelengths at a specified induced voltage value will not become the same at all the grid positions with respect to the scale 12. That is, as shown in (b) of FIG. 7, at the positions Xa and Xb at which the induced voltage values V1a of the pitch xcex1 are the same, the induced voltage values of the pitch xcex2 are not identical to each other. Therefore, by converting the relationship in the induced voltage between the two wavelengths into the positions, the absolute position of the grid can be detected.
Thus, the induction type transducer can measure dimensions with high accuracy even in an environment with a relatively high degree of pollution, however, it is necessary that a plurality of exciting coils and detecting coils are formed on the grid, and in particular, when the transducer detects an absolute position, the structure of the grid becomes complicated, and the transducer increases in size. Furthermore, when such an induction type transducer is built-in an electronic caliper, an increase in size of the transducer leads-in an increase in size of the electronic caliper itself, and lowering of workability when measuring.
The invention is made in view of the abovementioned problems in the related art, and the object thereof is to provide a small-sized induction type transducer with high performance, and furthermore, an electronic caliper using such a small-sized magnetic type transducer with high performance.
In order to achieve the abovementioned object, an induction type transducer of the invention, which outputs an electric signal in accordance with relative displacement between two members, comprises a magnetic flux generating section for generating a magnetic flux based on a drive signal, a magnetic flux detecting section for detecting a magnetic flux which changes in accordance with the relative positions, and a signal processing section for processing a detection signal from the magnetic flux detecting section. In the induction type transducer, the magnetic flux generating section, magnetic flux detecting section, and signal processing section form, a multilayer structure. The magnetic flux generating section, magnetic flux detecting section, and signal processing section are not disposed in parallel on the same plane, but are formed on respective layers in the multilayer structure, whereby the transducer can be reduced in size.
Herein, the multilayer structure is preferably a structure in which a plurality of layers are built-up on a core layer. The use of the built-up substrate reduces the size, thickness, and weight. Furthermore, the number of layers in the multilayer structure becomes easily properly adjustable.
Furthermore, the magnetic flux generating section is formed at the measuring plane side and the signal processing section is formed at the opposite side of the measuring plane. Herein, the measuring plane side means the side opposed to the scale. By forming the magnetic flux generating section at the measuring plane side, a generated magnetic flux can be made to effectively influence the measuring side. Also, by forming the signal processing section at the opposite side of the measuring plane, the influence of an unnecessary magnetic flux on the signal processing section and a mixture of electromagnetic noise can be prevented.
Furthermore, it is preferable that the magnetic flux generating section and the magnetic flux detecting section are formed at the measuring plane side, and the signal processing section is formed at the opposite side of the measuring plane. The abovementioned effect can be obtained even when the magnetic flux detecting section is formed within the same plane as with the magnetic flux generating section.
Furthermore, it is preferable that the magnetic flux generating section is formed at a position closer to the measuring plane than the magnetic flux detecting section and the signal processing section in the multilayer structure, and the magnetic flux detecting section is formed at a position closer to the measuring plane than the signal processing section in the multilayer structure. Thereby, a magnetic flux generated from the magnetic flux generating section tan be made to effectively influence the measuring plane side, and an induced magnetic field is effectively detected by the magnetic flux detecting section and the signal processing section is separated from the magnetic coupling range, whereby the mixture of unnecessary electromagnetic noise can be prevented. The magnetic flux detecting section may be formed at a position closer to the measuring plane than the magnetic flux generating section and the signal processing section in the multilayer structure, and the magnetic flux generating section may be formed at a position closer to the measuring plane than the signal processing section in the multilayer structure.
Furthermore, it is preferable that at least one magnetic shield section is formed between the magnetic flux generating section and the signal processing section in the multilayer structure. In a case where the magnetic flux generating section and the signal processing section are formed at respective layers of the multilayer structure, since both sections become close in distance to each other, a magnetic flux generated by the magnetic flux generating section may directly influence the signal processing section, and signals other than an original detection signal may mix into the signal processing section due to, the change in the magnetic flux. Therefore, by providing a magnetic shield section, the mixture of such electronic noise can be suppressed and the detection accuracy can be improved. The magnetic shield section is preferably formed at one layer of the multilayer structure, and a single magnetic shield section or a plurality of magnetic shield section may be provided at different layers. The magnetic shield section can be formed from at least any of nonmetals with high magnetic permeability represented by ferrite, metals with low magnetic permeability represented by copper, and metals with high magnetic permeability represented by permalloy. When the magnetic shield section is formed from metal, in order to prevent deterioration in the signal strength due to capacitive coupling between the magnetic flux generating section and the magnetic flux detecting section, it is preferable that the magnetic shield section is maintained at a constant voltage, for example, a voltage at ground level.
When the magnetic shield section is formed from metal, the magnetic shield section is preferably formed so as to be separated from the magnetic flux generating section or magnetic flux detecting section by a distance equivalent to or longer than the gap between the two members. The magnetic shield section functions as a shielding means for suppressing the influence of a magnetic flux from the magnetic flux generating section on the signal processing section. However, if the magnetic shield section is formed from metal with low magnetic permeability such as copper, which is easily acquired, an induced current (eddy current) is generated in the magnetic shield section due to the magnetic flux from the magnetic flux generating section, and this eddy current tends to cancel the magnetic flux, so that the signal strength deteriorates. Therefore, in the case where the magnetic shield section is formed so as to be separate by a predetermined distance from the magnetic flux generating section or the magnetic flux detecting section it the magnetic flux generating section is formed at the same layer as that of the magnetic flux detecting section, the deterioration in signal strength can be suppressed and the detection accuracy can be improved. It is desirable that the distance between the magnetic flux generating section or magnetic flux detecting section and the magnetic shield section is determined in accordance with the distance between the two members, more specifically, the gap (air gap) between the magnetic flux generating section and other member in terms of detection accuracy. Even when the magnetic flux density in the measuring plane lowers due to the eddy current generated in the magnetic shield section, if the gap is sufficiently small, the detection signal strength is maintained. By separating the magnetic shield section from the magnetic flux generating section or magnetic flux detecting section by a distance equivalent to or longer than the gap, attenuation of the detection signal strength can be suppressed. If the magnetic shield section is excessively separated, the thickness of the multilayer structure increases accordingly, so that this is not suitable for practical use.
When the magnetic shield section is formed from nonmetal, it is preferable that the magnetic shield section is formed at a close distance from the magnetic flux generating section or magnetic flux detecting section that is equivalent to or shorter than the gap between the two members. When a nonmetal with high magnetic permeability such as ferrite is used for the magnetic shield section, the magnetic flux density is not reduced so much by an eddy current, and the magnetic shield section is disposed to be close to the abovementioned member and the magnetic flux density can be increased due to its high magnetic permeability.
The abovementioned induction type transducer can be applied to, for example, an electronic caliper, whereby the electronic caliper can be reduced in size and improved in performance.
In the electronic caliper, the induction type transducer can be built-in at the grid (slider) side, however, the gap between the grid and scale is preferably set to be approximately one tenth of the pitch of the detection signal. If the gap is too large, the amount of magnetic flux reaching the scale decreases, and if the gap is too small, the influence of the form of the coils at the scale side increases and the detection signal is greatly distorted. Therefore, there is an optimum size of the gap whereby the amount of reduction in magnetic flux reaching the scale is made small (by reducing the gap size to some degree), and the distortion in the detection signal is made small (by increasing the gap size to some degree), and concretely the size of gap of approximately one tenth of the pitch (or wavelength) of the detection signal is optimum in terms of the magnetic flux and signal distortion. Thereby, the detection accuracy can be further improved.